Language proposal: dotnet/csharplang#3194
General compiler test plan: https://github.com/dotnet/roslyn/blob/master/docs/contributing/Compiler%20Test%20Plan.md

Spec:

• Complete proposal spec is checked into csharplang

Compiler (Both sync and async for each of these):

• [x] LangVer (TestGetEnumeratorPatternViaExtensionsCSharp8, )
• [x] Feature spec exists and reviewed with LDM
• [x] Update compiler test plan
• [x] IOperation/CFG
  • [x] CFG shows the extension method getting called
• [ ] GetForEachStatementInfo
• [x] Differing access modifiers
  • [x] Extension GetEnumerator should work for internal, unlike regular GetEnumerator (TestGetEnumeratorPatternViaInternalExtensions, TestGetAsyncEnumeratorPatternViaInternalExtensions)
  • [x] Test with internal instance method and extension method on the same type (TestGetEnumeratorPatternViaExtensionWithInternalInstanceGetEnumerator, TestGetAsyncEnumeratorPatternViaExtensionWithInternalInstanceGetAsyncEnumerator)
  • [x] private extension methods when inside a static class (TestGetEnumeratorPatternViaAccessiblePrivateExtension, TestGetAsyncEnumeratorPatternViaAccessiblePrivateExtension)
• [x] out/ref disallowed.
  • [x] out (, )
  • [x] ref (TestGetEnumeratorPatternViaRefExtensionOnNonAssignableVariable, TestGetAsyncEnumeratorPatternViaExtensionWithParams)
• [x] in allowed in the same places as Deconstruct. (TestGetEnumeratorPatternViaInExtensionOnNonAssignableVariable, TestGetAsyncEnumeratorPatternViaInExtensionOnAssignableVariable)
• [x] Default parameters (TestGetEnumeratorPatternViaExtensionWithOptionalParameter, TestGetAsyncEnumeratorPatternViaExtensionWithOptionalParameter)
• [x] params arrays (TestGetEnumeratorPatternViaExtensionWithParams, TestGetAsyncEnumeratorPatternViaExtensionWithParams)
• [x] __arglist parameter (TestGetEnumeratorPatternViaExtensionWithArgList, TestGetAsyncEnumeratorPatternViaExtensionWithArgList)
• [x] dynamic prefers existing attempt to call instance GetEnumerator, not an extension method, or does not work on async (TestPreferIEnumeratorInterfaceOnDynamicThanViaExtension, TestCannotUseExtensionGetAsyncEnumeratorOnDynamic)
• [x] Conversions
  • [x] Reference conversions (TestGetEnumeratorPatternViaExtensionsWithUpcast, TestGetAsyncEnumeratorPatternViaExtensionsWithUpcast)
  • [x] Boxing conversion (TestGetEnumeratorPatternViaExtensionsWithNullableValueTypeConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithNullableValueTypeConversion)
  • [x] Unboxing (should show helpful error, like for regular invocation) (TestGetEnumeratorPatternViaExtensionsWithUnboxingConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithUnboxingConversion)
  • [x] Literal 0 to enum (same as Unboxing) (TestGetEnumeratorPatternViaExtensionsWithZeroToEnumConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithZeroToEnumConversion)
  • [x] Unconstrained generics (TestGetEnumeratorPatternViaExtensionsWithUnconstrainedGenericConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithUnconstrainedGenericConversion)
  • [x] Constrained generics (TestGetEnumeratorPatternViaExtensionsWithConstrainedGenericConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithConstrainedGenericConversion)
  • [x] Interpolated string conversions (IFormattableString vs object) (TestGetEnumeratorPatternViaExtensionsWithFormattableStringConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithFormattableStringConversion1)
  • [x] User-defined conversions (should be invalid) (TestGetEnumeratorPatternViaExtensionsWithUserDefinedImplicitConversion, TestGetAsyncEnumeratorPatternViaExtensionsWithUserDefinedImplicitConversion)
  • [x] Tuple type conversions (TestGetEnumeratorPatternViaExtensionsOnTupleWithNestedConversions, TestGetAsyncEnumeratorPatternViaExtensionsOnTupleWithNestedConversions)
• [x] this types:
  • [x] class (a lot)
  • [x] interface (TestGetEnumeratorPatternViaExtensionsOnInterface, TestGetAsyncEnumeratorPatternViaExtensionsOnInterface)
  • [x] delegate (TestGetEnumeratorPatternViaExtensionsOnDelegate, TestGetAsyncEnumeratorPatternViaExtensionsOnDelegate)
  • [x] struct (, TestGetAsyncEnumeratorPatternOnRange)
  • [x] enum (TestGetEnumeratorPatternViaExtensionsOnEnum, TestGetAsyncEnumeratorPatternViaExtensionsOnEnum)
  • [x] nullable (TestGetEnumeratorPatternViaExtensionsOnNullable, TestGetAsyncEnumeratorPatternViaExtensionsOnNullable)
  • [x] type parameters (TestGetEnumeratorPatternViaExtensionsOnTypeParameter, TestGetAsyncEnumeratorPatternViaExtensionsOnTypeParameter)
• [x] Ambiguous overloads (TestGetEnumeratorPatternViaAmbiguousExtensions, TestGetAsyncEnumeratorPatternViaAmbiguousExtensions)
• [x] Correct name signature, but return doesn't fulfill the pattern (, TestMoveNextAsyncPatternViaExtensions1)
• [x] Extension is preferred last (TestPreferEnumeratorPatternFromInstanceThanViaExtension, TestPreferAsyncEnumeratorPatternFromInstanceThanViaExtension)
• [x] If GetEnumerator is defined on the class, extensions are not searched, even if return is incorrect. (TestPreferEnumeratorPatternFromInstanceThanViaExtensionEvenWhenInvalid, TestPreferAsyncEnumeratorPatternFromInstanceThanViaExtensionEvenWhenInvalid)
• [x] Multiple extensions in different namespace levels
  • [x] Valid overload in closest namespace (with invalid in outer namespace) (TestGetEnumeratorPatternViaValidExtensionInClosestNamespaceInvalidInFurtherNamespace1-2, TestGetAsyncEnumeratorPatternViaValidExtensionInClosestNamespaceInvalidInFurtherNamespace1-2)
  • [x] Invalid overload in closest namespace (with valid in outer namespace) (TestGetEnumeratorPatternViaInvalidExtensionInClosestNamespaceValidInFurtherNamespace1-2, TestGetAsyncEnumeratorPatternViaInvalidExtensionInClosestNamespaceValidInFurtherNamespace1-2)
• [x] ref returns (TestGetEnumeratorPatternViaExtensionWithRefReturn, TestGetAsyncEnumeratorPatternViaExtensionWithRefReturn)
• [x] Nullable warnings
  • [x] No warning for extension method accepting null (ForEach_ExtensionGetEnumerator6, ForEach_ExtensionGetAsyncEnumerator6)
  • [x] Warning for extension method not accepting null (ForEach_ExtensionGetEnumerator1, ForEach_ExtensionGetAsyncEnumerator5)
  • [x] Constraint mismatch warnings (ForEach_ExtensionGetEnumerator3, ForEach_ExtensionGetAsyncEnumerator3)
  • [x] Warning for nullable return (ForEach_ExtensionGetEnumerator15, ForEach_ExtensionGetAsyncEnumerator15)
  • [x] Output attributes are honored after call (ForEach_ExtensionGetEnumerator7-8, ForEach_ExtensionGetAsyncEnumerator7)
• [x] Obsolete extension methods (TestGetEnumeratorPatternViaObsoleteExtension, TestWithObsoletePatternMethodsViaExtension)
• Disposal
  • [x] GetEnumerator() returning IDisposable types (TestForEachViaExtensionExplicitlyDisposableStruct, TestAwaitForEachViaExtensionExplicitlyDisposableStruct)
  • [x] Uses pattern-based Dispose method on async, not on sync (TestForEachViaExtensionDisposeStruct, TestAwaitForEachViaExtensionAsyncDisposeStruct)
  • [x] Uses pattern-based for both for ref structs (, TestAwaitForEachViaExtensionImplicitlyDisposableStruct)

Productivity:

• [ ] test FindAllReferences
• [x] AddParameter to a GetEnumerator
• [ ] AddUsing to bring extension GetEnumerator into scope
• [ ] foreach to Linq
• [ ] Linq to foreach